Woon-Seop Choi

Effect of the hydrophobicity and thickness of polymer gate dielectrics on the hysteresis behavior of pentacene-based field-effect transistors

We demonstrate the origin and mechanism of the hysteresis behavior that is frequently observed during the operation of organic field-effect transistors (OFETs) based on polymer gate dielectrics. Although polar functionalities, such as hydroxyl groups, present in the polymer gate dielectrics are known to induce hysteresis, there have only been a few detailed investigations examining how the presence of such end functionalities both at the polymer surface—forming an interface with the semiconductor layer—and in the bulk influences the hysteresis. In this study, we control the hydrophobicity of the polymer by varying the number of hydroxyl groups, and use an ultrathin polymer/SiO2 bilayer and a thick single polymer as the gate dielectric structure so that the hysteresis behavior is divided into contributions from hydroxyl groups present at the polymer surface and in the bulk, respectively. Electrical characterizations of the OFETs, performed both in vacuum (≈10−3 Torr) and in ambient air (relative humidity o...

Inkjet-Printed In2O3 Thin-Film Transistor below 200 °C

High-performance In2O3 thin-film transistors could be prepared by an inkjet-printing method below 200 °C with a single precursor and solvent formulation. The self-combustion reaction took place with the electrical properties of In2O3 at a low temperature of 147 °C, which was confirmed by X-ray photoelectron spectroacopy and thermal analysis. The electrical properties after postannealing at 200 °C were as follows: a mobility of 3.98 cm(2)/V·s, a threshold voltage of 1.83 V, a subthreshold slope of 0.4 V/dec, and an on-to-off current ratio of 10(8), which are the best properties by an inkjet process thus far. The positive bias stability was much improved by postannealing, and good negative bias stability was obtained.

Solution-processed flexible ZnO transparent thin-film transistors with a polymer gate dielectric fabricated by microwave heating

We report the development of solution-processed zinc oxide (ZnO) transparent thin-film transistors (TFTs) with a poly(2-hydroxyethyl methacrylate) (PHEMA) gate dielectric on a plastic substrate. The ZnO nanorod film active layer, prepared by microwave heating, showed a highly uniform and densely packed array of large crystal size (58 nm) in the [002] direction of ZnO nanorods on the plasma-treated PHEMA. The flexible ZnO TFTs with the plasma-treated PHEMA gate dielectric exhibited an electron mobility of 1.1 cm(2) V(-1) s(-1), which was higher by a factor of approximately 8.5 than that of ZnO TFTs based on the bare PHEMA gate dielectric.

Hysteresis behaviour of low-voltage organic field-effect transistors employing high dielectric constant polymer gate dielectrics

Here, we report on the fabrication of low-voltage-operating pentacene-based organic field-effect transistors (OFETs) that utilize crosslinked cyanoethylated poly(vinyl alcohol) (CR-V) gate dielectrics. The crosslinked CR-V-based OFET could be operated successfully at low voltages (below 4 V), but abnormal behaviour during device operation, such as uncertainty in the field-effect mobility (μ) and hysteresis, was induced by the slow polarization of moieties embedded in the gate dielectric (e.g. polar functionalities, ionic impurities, water and solvent molecules). In an effort to improve the stability of OFET operation, we measured the dependence of μ and hysteresis on dielectric thickness, CR-V crosslinking conditions and sweep rate of the gate bias. The influence of the CR-V surface properties on μ, hysteresis, and the structural and morphological features of the pentacene layer grown on the gate dielectric was characterized and compared with the properties of pentacene grown on a polystyrene surface.

The Fabrication of Tin Oxide Films by Atomic Layer Deposition using Tetrakis(Ethylmethylamino) Tin Precursor

films have been fabricated using many different technologies, including sputtering [1], chemical vapor deposition [2], spray pyrolysis [3], and atomic layer deposition (ALD) [4], [5]. Amongst these ALD produces unique film properties compared with other methods. In particular dense films of high aspect ratio and fewer defects are achieved. ALD is also an ideal technology for the deposition of ultrathin films with high conformality and precise thickness control [6]. ALD is a dense thin film growth technology using an alternating self-controlled chemical process between the gas precursor and a solid surface in order to deposit materials in an atomic layer-by-layer fashion. This methodology can produce films with very precise control over film thickness and composition. This technology allows precise coatings to be applied on other surfaces of powder and porous materials. Plasma-enhanced ALD (PEALD) is a promising technology for obtaining high quality metal oxide films at relatively low temperatures compared with commercial CVD. Interestingly, the comparative study of the growth of tin oxide films by ALD and chemical vapor deposition was reported [7]. The films were of the tetragonal rutile-type for both cases. Thin films grown by ALD were found to be close to being perfect single crystals, containing a low density of defects and were almost atomically smooth. The CVD films had a much rougher film morphology and exhibited both grain boundaries and twin formation. Tin oxide thin films have been fabricated previously by ALD using halogenated precursors, SnCl

Electrohydrodynamic Jet-Printed Zinc–Tin Oxide TFTs and Their Bias Stability

Zinc-tin oxide (ZTO) thin-film transistors (TFTs) were fabricated using an electrohydrodynamic-jet (EHD-jet) printing technique at annealing temperatures ranging from 300 to 500 °C. An EHD-jet-printed ZTO active layer was patterned with a 60 μm width using a 100 μm inner diameter metal nozzle. The electrical properties of an EHD-jet-printed ZTO TFT showed a mobility of 9.82 cm(2)/(V s), an on-off current ratio of 3.7 × 10(6), a threshold voltage of 2.36 V, and a subthreshold slope of 0.73 V/dec at 500 °C. Significantly improved properties were obtained compared to the spin-coated and inkjet-printed ones. Better hysteresis behavior and positive bias stability of the ZTO TFTs were also achieved using EHD-jet printing technology.

Recent advances in salivary cancer diagnostics enabled by biosensors and bioelectronics.

There is a high demand for a non-invasive, rapid, and highly accurate tool for disease diagnostics. Recently, saliva based diagnostics for the detection of specific biomarkers has drawn significant attention since the sample extraction is simple, cost-effective, and precise. Compared to blood, saliva contains a similar variety of DNA, RNA, proteins, metabolites, and microbiota that can be compiled into a multiplex of cancer detection markers. The salivary diagnostic method holds great potential for early-stage cancer diagnostics without any complicated and expensive procedures. Here, we review various cancer biomarkers in saliva and compare the biomarkers efficacy with traditional diagnostics and state-of-the-art bioelectronics. We summarize biomarkers in four major groups: genomics, transcriptomics, proteomics, and metabolomics/microbiota. Representative bioelectronic systems for each group are summarized based on various stages of a cancer. Systematic study of oxidative stress establishes the relationship between macromolecules and cancer biomarkers in saliva. We also introduce the most recent examples of salivary diagnostic electronics based on nanotechnologies that can offer rapid, yet highly accurate detection of biomarkers. A concluding section highlights areas of opportunity in the further development and applications of these technologies.

Electrohydrodynamic Jet Spraying Technique for Oxide Thin-Film Transistor

Electrohydrodynamic (EHD) jet is a printing technique using an electric field to create jetting droplets for the delivery of a liquid portion to a designated substrate. EHD jet spraying was applied to the preparation of a solution-processed zinc-tin-oxide thin-film transistor. The electrosprayed film was characterized by analytical methods. Electrical properties, such as a mobility of 4.89 cm²·V·s^-1, a threshold voltage of 7.17 V, a subthreshold slope of 0.44 V·dec^-1, and an on-to-off ratio of 10⁷, were obtained.

Effect of aluminum doping on a solution-processed zinc-tin-oxide thin-film transistor

The doping effect of aluminum (Al) on a solution-processed zinc-tin-oxide (Al-ZTO) film was examined. The solution processing temperature and aluminum doping concentration were optimized. The following optimal electrical properties with 0.0025 M Al doping at a processing temperature of 500°C were obtained; a mobility of 5.41 cm2/Vs, a current ratio of Ion/Ioff of 107, a threshold voltage of 2.46 V, and a subthreshold slope of 0.48 V/dec with better bias stability. Al doping into the ZTO TFTs improved the electrical properties and bias stability due to the control of free charge carriers without suppressive atoms.

Interfacial study of metal oxide with source-drain electrodes and oxide semiconductor by XPS

As an interlayer for source-drain electrodes in solution processed zinc tin oxide (ZTO) thin-film transistor, MoO3 was introduced between aluminum source-drain electrodes and oxide semiconductor. MoO3 was detected not only in the interlayer, but in the source-drain electrodes and oxide semiconductor layer. The chemical configuration and the structural configuration were confirmed by XPS interfacial study and by TEM analysis, respectively. From these analytical tools, we found that the interlayer exists as a chemically mixed state between the layers, source-drain electrodes and oxide semiconductor.